Metal pipes often have their outer surfaces covered with a protective coating. These pipes are used for conveying brake fluids, fuel and the like in a motor vehicle. As such, these pipe lines are located under the body of the vehicle. Since they are used in such a harsh environment, the pipes are required to have a high degree of corrosion resistance, scratch resistance, impact strength and mechanical wear resistance. In cold climates, it is not unusual to encounter rock salt sprinkled onto road surfaces in order to prevent freezing of water on the road surfaces and the inherent dangers caused thereby. The popularity of spreading rock salt has created a serious problem of pipe corrosion. The pipes are also vulnerable to damage or wear from stones or mud spattered by rotating wheels of the vehicle. It is, therefore, necessary that the pipes attached to the underbody of the vehicle be coated so as to resist both chemical corrosion and mechanical damage or wear.
A double-rolled steel pipe has been proposed that is made by rolling a steel strip or hoop twice and brazing its longitudinal edges by means of a copper plating layer, or a seam welded steel pipe, where the pipe has an outer surface coated with an electroplated zinc film. The zinc film has an outer surface coated with a relatively thick special chromate film having an olive color. The chromate film has an outer surface coated with a fluorinated resin film. The fluorinated resin film is formed by impregnating the chromate film with a dispersion of polyvinyl fluoride immediately after the formation of the chromate film when it is still in the state of a gel, and drying them under heat, so that the fluorinated resin film may form an intimate bond with the chromate film. When the chromate film is formed by treating the pipe with a solution, it requires large amounts of a chromium compound and an organic acid, such as formic acid, used as a reducing agent. It is necessary to supply the treating solution with the chromium compound frequently, and to renew it at regular intervals of time in order to maintain a constant film forming capacity. The waste solution, however, contains a large amount of chromium having a valence of 6, which is a toxic substance, and its disposal, therefore is very costly. Although the chromate film as formed is highly resistant to corrosion, the heat to which it is exposed during the formation of the resin film deprives it of water and thereby makes it brittle. Any plastic deformation of the pipe, such as the result of bending or double flaring, forms fine cracks in the chromate film which lowers its rustproofing properties.
It has also been proposed to provide a corrosion resistant pipe where a metal pipe is provided with an outer surface coated with a zinc film, a chromate film, an intermediate layer consisting sequentially of an epoxy resin, and a polyvinyl fluoride film formed one on top of another in the order listed. A plastic-coated steel tube has also been proposed where a steel tube has an inner layer of at least one cross-linked polyolefin modified with a hydrolyzable silane and an outer unmodified or soot-blended polyolefin layer on the exposed surface of the inner layer. A process for coating metal tubes with plastic material has also been disclosed where a fixed metal tube is heated to a temperature above the melting point of the plastic material to be employed, thereafter causing a mixture of plastic powder and air to pass through the metal tube whereby the plastic material is fritted onto the inside surface of the tube, thereafter rotating the metal tube and applying to the exterior surface thereof in a plurality of stages a plastic material, the plastic material being electrostatically sprayed onto the rotating metal tube and after each stage of electrostatically applying plastic to the outside surface of the metal tube and applying plastic material to the inside surface thereof, completely melting and smoothing the plastic material.
An automobile tube line for a brake, fuel or hydraulic system has also been disclosed with an interior steel tube having a galvanized exterior layer with an additional exterior olive chromated layer which is wrapped in an additional Nylon 12 layer casing where the plastic casing is a polyamide layer applied by extrusion on top of the olive chromated layer.
It is also important that the tubing stock material of choice be essentially non-reactive with the materials conveyed within. Various fluids conveyed through the tubing exhibit degrees of reactivity with interior tubing surfaces. For instance, the components of various alternate fuels for example methanol contained in fuels such as M85 and ethanol in fuels such as E85 can react with metals such as low carbon steel to cause corrosion. The resulting corrosion can weaken the tubing; eventually resulting in rupture, leakage and premature failure. Addition, even materials which are not directly corrosive in a metallic environment can render the metal susceptible to exterior galvanic corrosion, Thus, the useful life of conventional low carbon steel is shortened even when conveying only mildly reactive materials.
Reactivity between the tubing and the materials conveyed within it can also compromise the purity of the conveyed fluid materials. Because of this, inexpensive metal tubing cannot be used in a variety of applications where possible fluid contamination with metal complexes, liquids or the like are not acceptable. In such instances, it is necessary to subject the tubing to post-formation processes such as annealing or to resort to more expensive tubing made of non-reactive metals or to tubing made from polymeric materials.
Polymeric tubing has been suggested as a substitute for metal walled tubing in various instances to eliminate the problems such as those previously discussed. Unfortunately, polymeric tubing presents a different set of problems. Conventional polymeric tubing is generally composed of materials such as polypropylene, polyvinyl chloride and the like. These polymers exhibit poor formability characteristics. Because the polymeric materials exhibit elastic memory, the tubing constructed from such materials is difficult to permanently contour.
Prolonged use of sections of polymeric tubing without metal walls can lead to the development of static charge. In this phenomenon, static charge builds up along the plastic line and, ultimately, results in numerous, unpredictable pinhole ruptures in the tubing. As can be appreciated, the use of polymeric lines is not desirable in high pressure applications or in situations in which the tubing will convey flammable liquids which could ignite upon escape during rupture. Furthermore, it is difficult to achieve burst strength characteristics in monolithic structures even if problems of static discharge can be obviated.
Even if problems of formability and static discharge can be overcome, non-metal walled polymeric tubing, exhibits extreme weakness to heat. The polymeric materials employed in conventional plastic tubing sag, weaken or melt at undesirably low temperatures; rendering them impractical for many applications such as use in conjunction with automobile engines.
Finally, polymeric materials which make up the non-metal walled tubing can interact with certain organic components causing softening, localized deformation of the tubing, or permeation of the organic material through the polymeric material. Thus, a variety of organic fluids cannot be conveyed through polymeric tubing stock.
The use of composite or lined tubing stock having an outer metal layer and an interior non-reactive polymeric layer has been proposed. However, to date, metal tubes having relatively wide inner diameters have been produced by spraying a liquid polymeric material onto the interior of the metal tube by means of an appropriate spray nozzle or other suitable dispensing device inserted into the interior of the metal tube to deliver molten polymeric material directly onto the interior wall. This method generally limits the production of such tubing to tubing stock having an inner diameter of sufficient width to permit insertion of the polymer dispensing device. Because the polymer applicator can only be inserted a relatively short distance into the metal tube, the overall length of lined tubing produced by this method is limited. Given these constraints, it can be appreciated that it has been impossible to employ the method to small-diameter tubing. Furthermore, it has been difficult to assure that the applied polymer is uniformly dispersed over the interior surface of the tubing. Non-uniformities of the polymer deposited on the interior surface of the tubing can result in unwanted narrowing of inner diameter of the tubing causing constriction or blockage or insufficient polymer coating to prevent interaction with the reactive or corrosive fluids conveyed therethrough.
In co-pending application Ser. No. 08/260,523, filed on Jun. 16, 1994, there has been proposed a lined small diameter tubing and method of making the same which has a diameter less than about 1 inch with diameters between 3/16 and 3/4 inch being preferred. In this invention, the composite tube comprises a cylindrical outer metal conduit having an outer wall and an interiorly oriented wall which defines a central shaft extending longitudinally therethrough and an interior tube made of a suitable non-reactive material such as a polymeric material surrounded by and positioned within the cylindrical outer metal conduit. The outer wall surface of the interior tube matingly engages the interiorly oriented wall of the metal conduit and conforms to the interiorly oriented wall to protect the surface of the interiorly oriented metal wall from contact with potentially reactive fluids conveyed therethrough. The tubing described therein lacks an effective means to protect the exterior tube surface from environmental degradation and corrosion.